Why does Thanksgiving dinner make you sleepy?

Janet D. Stemwedel is an Associate Professor of Philosophy at San José State University. Her explorations of ethics, scientific knowledge-building, and how they are intertwined are informed by her misspent scientific youth as a physical chemist. Follow on Twitter @docfreeride.

Janet D. Stemwedel is an Associate Professor of Philosophy at San José State University. Her explorations of ethics, scientific knowledge-building, and how they are intertwined are informed by her misspent scientific youth as a physical chemist. Follow on Twitter @docfreeride.

For years, you’ve heard the tremendous fatigue experienced after an American Thanksgiving dinner laid at the feet of the turkey — or more precisely, blamed upon the tryptophan in that turkey. Trytophan, apparently, is the go-to amino acid for those who want to get sleepy.

Let me note, before we go on, that for all its association with tryptophan, turkey doesn’t even crack the top 50 in this list of tryptophan-rich foods. (Number one: stellar sea lion kidney.)

Turkey does contain a large amino acid called tryptophan. So eating turkey puts some tryptophan into your bloodstream. But there are lots of other large amino acids riding around in there too.

For the tryptophan in turkey to do its sleep-inducing work, it needs an accomplice. Maybe the bread stuffing, the mashed potatoes, those candied yams:

When you eat carbohydrates, the pancreas releases insulin, and one effect of that is to lower the levels of all the large amino acids in your blood — except for tryptophan. The upshot? You have relatively high levels of tryptophan in your blood, and in your brain that’s converted into the neurotransmitter serotonin, and that can make you sleepy.

It seems that the turkey’s tryptophan dose is amplified by the sweet and starchy sides. Indeed, perhaps the sides alone would do the job (by clearing out the non-tryptophan amino acids) even if you missed out on the turkey.

(We shouldn’t forget, of course, that eating more than you’re used to in a sitting — and giving your system more digestive work to do than it’s used to — might account for a good bit of the fatigue.)

In fact, the article suggests that maybe you’ll be even more sleepy if you don’t eat your turkey:

[E]ating protein has the opposite effect from eating carbohydrates — it raises the blood levels of all large amino acids. If all you ate were turkey, you’d have relatively low levels of tryptophan — and, if anything, you’d have some extra get-up-and-go, instead of all that extra lie-down-and-snooze.

Let’s pull back a moment to get clear on the tryptophan theory of needing a nap between dinner and dessert. Is the drowsiness due to the level of tryptophan relative to the other amino acids kicking around in your bloodstream? Or is it due to the the level of tryptophan relative to how much is normally present in your bloodstream?

If the article got the scientific story right, it’s the former — so boosting non-tryptophan large amino acids would counteract the yawns, as would taking in less tryptophan. (I’m guessing maybe the tryptophan-uptake apparatus is sampling from the available large amino acids, which would mean whatever the absolute level of tryptophan coursing through your veins, a high relative level of tryptophan (compared to the other amino acids in the mix) is going to trip the “boy are you sleepy!” circuits more often than not. The brain chemistry mavens are invited to chime in with the relevant facts.)

In any case, this whole discussion seems like a perfect opportunity to conduct some citizen science (and, come Friday, to collect some reports from the field).

Ideally, we’d all want to sit down to the same Thanksgiving meal together (having all gotten a good night’s sleep the day before, etc., etc.). Sadly, that’s not going to happen. However, maybe you can rope those with whom you are dining on Thursday into participating.

Depending on the vibe at your Thanksgiving table, you can either ask the diners to keep track of what kinds of foods they eat, or you can assign your guests particular consumption objectives. Then, before dessert, have everyone do a quick assessment of his or her energy level.

Of course, if you track participant input a bit more precisely, maybe we’ll stumble upon some other factor that turns out to be important, like vitamin A or sage.

If you use my form, you can return your results to me (as a *.doc or scanned into a PDF) by email: dr – dot – freeride – at – gmail – dot – com. I’ll compile the responses and we’ll see if we can make sense of the data.

See you back here on Friday morning with your results!

About the Author: Janet D. Stemwedel is an Associate Professor of Philosophy at San José State University. Her explorations of ethics, scientific knowledge-building, and how they are intertwined are informed by her misspent scientific youth as a physical chemist. Follow on Twitter @docfreeride.

3 Comments

I think you nailed it with this: “We shouldn’t forget, of course, that eating more than you’re used to in a sitting — and giving your system more digestive work to do than it’s used to — might account for a good bit of the fatigue.” People over-indulge during Thanksgiving dinner. Digesting carbs and (in particular) fats takes a lot of energy. To account for this variable, you should include other criteria in your test: the same combinations in your list, but with much smaller portions.